docs: def _setting(self, key, default):
          """Return the setting, checking config, then the appropriate
          environment variable, falling back to the default, caching the
          results.
  
          :param str key: The key to get
          :param any default: The default value if not set
          :return: str
  
          """
          if key not in self._settings:
              value = self._settings_in.get(
                  key, os.environ.get('STATSD_{}'.format(key).upper(), default))
              self._settings[key] = value
          return self._settings[key]